1. Field of the Invention
This invention, generally relates to electrochemical cells and, more particularly, to method for forming a carbon-hexacyanometallate battery electrode.
2. Description of the Related Art
A battery is an electrochemical cell through which chemical energy and electric energy can be converted back and forth. The energy density of a battery is determined by its voltage and charge capacity. Lithium has the most negative potential of −3.04 V vs. H2/H+, and has the highest gravimetric capacity of 3860 milli-amp-hours per gram (mAh/g). Due to their high energy densities, lithium-ion batteries have led the portable electronics revolution. However, the high cost of lithium metal renders doubtful the commercialization of lithium batteries as large scale energy storage devices. Further, the demand for lithium and its reserve as a mineral have raised the need to build other types metal-ion batteries as an alternative.
Lithium-ion (Li-ion) batteries employ lithium storage compounds as the positive (cathode) and negative (anode) electrode materials. As a battery is cycled, lithium ions (Li+) exchange between the positive and negative electrodes. Li-ion batteries have been referred to as rocking chair batteries because the lithium ions “rock” each and forth between the positive and negative electrodes as the cells are charged and discharged. The positive electrode (cathode) materials is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material having a tunneled structure, such as lithium manganese oxide (LiMn2O4), on an aluminum current collector. The negative electrode (anode) material is typically a graphitic carbon, also a layered material, on a copper current collector. In the charge-discharge process, lithium ions are inserted into, or extracted from interstitial spaces of the active materials.
Similar to the lithium-ion batteries, metal-ion batteries use the metal-ion host compounds as their electrode materials in which metal-ions can move easily and reversibly. As for a Li+-ion, it has one of the smallest radii of all metal ions and is compatible with the interstitial spaces of many materials, such as the layered LiCoO2, olivine-structured LiFePO4, spinel-structured LiMn2O4, and so on. Other metal ions, such as Na+, K+, Mg2+, Zn2+, etc., with large sizes, severely distort Li-based intercalation compounds and ruin their structures in several charge/discharge cycles. Therefore, new materials with large interstitial spaces would have to be used to host such metal-ions in a metal-ion battery.
FIG. 1 depicts the framework for an electrode material with large interstitial spaces in a metal-ion battery (prior art). It is inevitable that the large interstitial spaces in these materials readily absorb water molecules and impure ions, as shown. Water molecules also occupy lattices positions in the electrode material. Although these open spaces are very suitable for the intercalation of metal-ions with large sizes, the water molecules and impure ions degrade the electrochemical performance. In this example, Prussian blue analogues (PBs) with cubic/tetragonal/hexagonal framework have open “zeolytic” lattices that permit Na+/K+-ions to move easily and reversibly in the framework.
FIG. 2 demonstrates the crystal structure of Prussian blue and its analogues (prior art). Their general molecular formula is AM1M2(CN)6.zH2O, in which tetrahedrally coordinated A site is an alkali or alkaline-earth ion, and M1 and M2 are metal ions. The M1 and M2 metals are arranged in a three-dimensional checkerboard pattern and shown in a two-dimensional pattern. The crystal structure is analogous to that of the ABX3 perovskite. M1 and M2 metal ions are in ordered arrangement on the B sites. The M1 ions are octahedrally coordinated to the nitrogen ends of the CN— groups, and the M2 ions to their carbon ends. The M1 and M2 ions are connected by the C≡N to form the Prussian blue framework with large interstitial spaces.
The ratio of M1 and M2 may be an arbitrary number. The cyanide ligands (C≡N) octahedrally coordinate M1 and M2 to constitute a cubic framework that has a large interstitial space. The metal-ions or molecules of can locate in the interstitial space and balance the local charge. Although the molecular ratio for A:M1:M1:H2O in Prussian blue and its analogues is not precisely 1:1:1:0, the general molecular formula of AM1M2(CN)6is used herein for simplicity. The typical compounds of AM1M2(CN)6 include Prussian white (K2Fe(II)Fe(II)(CN)6), Prussian blue (KFe(II)Fe(III)(CN)6), Berlin green (Fe(III)Fe(III)(CN)6) and their analogues. The bond dipole moment of C≡N is around 3.0 Debye, which makes the ordering of the M1 and M2 ions with different spin-states in the framework. The material of AM1M2(CN)6 has demonstrated a variety of interesting functions in optics, magnetic, and electrochemistry.
In an electrochemical cell with AM1M2(CN)6 as an electrode component, the electrochemical reaction can take place only if (1) a redox couple is in the structure, (2) ions can transport in/out of the structure, and (3) electrons can transport to balance the charge neutrality. For example, the electrochemical reaction taking place in Prussian blue can be expressed as follows:KFe(II)Fe(III)(CN)6+K++e−→K2Fe(II)Fe(II)(CN)6.
In Prussian blue, the redox couple is Fe2+/Fe3+. In the reduction reaction, the Fe3+ ion obtains an electron and reduces to Fe2+, and a potassium-ion moves to the interstitial space of the compound to balance the charge. Once the ion diffusion or electron conduction becomes very slow, the reaction voltage departs from equilibrium (overpotential) and gives V=Veq−Vover, in which ‘Veq’ is the equilibrium potential of the electrochemical reaction and ‘Vover’ is the overpotential. Slower ion or electron transport leads to a high overpotential and a low reaction voltage. For a battery, its energy is E=V×I=(Veq−Vover)×I, where ‘I’ is the current. A large overpotential and small current lead to the poor performance of the electrochemical reaction.
The AM1M2(CN)6 material has a large interstitial space in which ions can easily move in and out of the lattice. Ion transport is not a limiting factor affecting the kinetic parameters of the electrochemical reaction. On the other hand, AM1M2(CN)6 has a wide band gap between the valence and conduction bands. This means that the AM1M2(CN)6 is a very poor electronic conductor. The dried Prussian blue, for example, is an insulator, and Prussian white and Berlin green are semiconductors. The slow electronic transport along the skeleton of M1-N≡C-M2 results in poor electrochemical performance in a system containing AM1M2(CN)6.
The large interstitial sites may host the large sized alkali or alkaline-earth ions (A). Water molecules may also occupy lattice positions in the PB analogues. The ion channels connecting the interstitial sites are similar in size to solvated alkali ions such as sodium, potassium, and alkaline-earth ions such as magnesium and calcium, allowing rapid transport of these ions throughout the lattice. Therefore, PB is a good choice for an electrode material in sodium/potassium/magnesium/calcium-ion batteries. Nonetheless, thermogravimetric analysis (TG) suggests that every PB molecule contains four to six water molecules. The occupation of water and impure ions in these materials definitely reduces the spaces to host the metal-ions and leads to the reduced capacity of these electrode materials. Therefore, KCuFe(CN)6 has a theoretical capacity of 85.2 mAh/g, but its practical capacity is smaller than 60 mAh/g. In addition, water may react with the intercalated metal-ions and decrease the coulombic and energy efficiencies of the metal-ion batteries. Up to now, no method has been reported to remove the water and impure ions from the large interstitial spaces and lattice positions of the hexacyanometallate electrode materials for metal-ions batteries. As a result, most metal-ions batteries with a hexacyanometallate electrode use an aqueous solution as an electrolyte. These batteries have small specific capacities and low voltages.
The open framework structure of the transition metal hexacyanometallates offers a faster and reversible intercalation process for alkali and alkaline-earth ions (Ax). In a metal-ion battery structure, the metal ions need to be stored in either the anode or cathode electrode before assembly. In the case of a Li-ion battery with LiCoO2, LiFePO4, and LiMn2O4 cathodes, the Li ions are stored in the cathode and the anode is carbon. Therefore, these batteries are assembled in a discharged state. These batteries need to be run through a charge cycle, to move the Li ions to the carbon anode, before they have any power for discharge. In the case of Li—S, Li-air and Na—S batteries, the metal ions are stored in anode. Actually, these anodes are made of Li and Na metals. These batteries are assembled in the charged state—meaning the battery can discharge immediately after assembly. Since alkali (e.g., Li, Na, and K), and other alkaline-earth (e.g., Mg and Ca) metals are very reactive with water vapor and oxygen, the manufacturing cost for such a battery would be prohibitively high, as the manufacturing has to be done in controlled environment.
In the case of sodium-ion batteries and potassium-ion batteries with hexacyanometallates AM1M2(CN)6 as the cathode materials, it is easy to use a metal anode for the metal-ion battery. For example, a Na-ion battery can be made of a sodium anode and KFe2(CN)6 cathode, or a K-ion battery with potassium anode and KFe2(CN)6 cathode. However, these batteries must be assembled in controlled environment (H2O-free, oxygen-free) if a metal anode is used.
It would be advantageous if electron transport could be improved in a AM1M2(CN)6 battery electrode.